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  • Writer's pictureZack Jurkowski

Why Do Old Floors Sag & Get Crooked?

The Basics: How Floors Are Built

The structure of a floor in a wood-frame home is comprised of a few key elements, and these are (from top to bottom):

  • Floor Covering: This is the resilient surface you walk on, like hardwood or parquetry.

  • Subfloor: This is what holds up your flooring. In newer homes, it's made of plywood, and in older homes it's made of thinner wooden planks

  • Joists: This is what holds up your subfloor. In newer homes it may be engineered i-joists, web trusses or dimensional lumber. In older homes, it is typically rough-sawn dimensional lumber. Joists for floors are like studs for walls.

  • Beam or Supporting Wall: This is what holds up your joists. In newer homes, these are typically steel or LVL (engineered beam). In older homes, they are typically built up with multiple layers of rough-sawn dimensional lumber, or steel. Occasionally, in much older homes, they may be made of something like one massive log of old-growth lumber.

Why Do Floors Get Crooked and Sag?

Wood construction homes are built using a system of joists and beams that support the floor structure. Over time, due to the weight of the building materials and the load imposed on the structure, the floor joists and beams can begin to deflect or sag in the middle. This deflection can be exacerbated by factors such as heavy loads beyond the original designed capacity, moisture, humidity, and temperature changes.

As the floor structure sags, the floor covering, which is typically made of hardwood, will also sag along with the structure. However, because hardwood is a relatively flexible material, it can follow the sagging structure slowly over time without causing damage to the floorboards.

This is because hardwood flooring is designed to be installed with a small gap between each board, which allows for natural expansion and contraction of the wood. As the structure sags, the hardwood floorboards will also flex and bend slightly, but they will not crack or break because they are able to move with the sagging structure.

How Do I Repair A Crooked Or Sagging Floor?

The proper way to address a sagging floor is to "sister" the joists. This is a process in which you add new joists to the old joists with glue and fasteners to both improve the rigidity of the floor, and raise the new joists so that your floor will be level again.

Are Crooked Floors A Sign Of Danger?

While old floors are almost certain to become slightly crooked or develop a sag over time, this isn't necessarily dangerous. The time to be concerned is if the sagging happens suddenly. A few questions to ask:

  • Has the sagging or crookedness appeared in a matter of weeks or days?

  • Are there gaps or cracks appearing in your floors or walls?

  • Are your windows or doors developing gaps or becoming hard to open or close?

  • Can you hear or feel movement in the structure that wasn't there yesterday?

If you answered yes to any of those questions, you should leave the building immediately and contact local authorities. In extreme cases, it may be necessary to contact 911 to perform and emergency inspection if you suspect that your home has suddenly become structurally compromised.

It is important to note that excessive sagging can cause other issues, such as uneven floors and gaps between the floorboards. Additionally, if the sagging becomes too severe, it can risk collapsing or it can cause damage to other parts of the building, such as the walls and ceiling. Therefore, it is important to monitor the condition of the floor structure and address any issues promptly to ensure the safety and integrity of the building.

If you're unsure of the condition of your floor, or if you're looking to get your old floor straightened out, contact us today for an immediate response and we'll be happy to help!


Montreal Contractors is a qualified company, holding an active bonded license as a General Contractor and Specialized Contractor with the Régie du Bâtiment du Québec (RBQ License: 5767-5480-01), ASP Health & Safety Certification and a $2M liability policy. Our employees have all passed background checks, are registered with CNESST and have their ASP Health and Safety certification.

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